The present invention relates to an apparatus for performing color image printing, and more particularly relates to a small-size color image printing apparatus utilizing an electro-photographic process.
In the past, there has been an electro-photographic printing apparatus utilizing electro-photography for printing a color image using image information from a computer or the like. The use of electro-photography has a disadvantage in that it calls for a complex structure, which is difficult to handle and is large in size, because of the large number of printing processes to be carried out therein. Particularly, if the apparatus is arranged to provide developing units for four colors arranged around a photosensitive drum, the apparatus becomes complex in that each of the developing units is required to change its structure and at the same time the diameter of the photosensitive drum becomes large. Therefore, in a conventional apparatus of this type, a photosensitive body having a comparatively small diameter is employed, and at the same time a switching method using a mechanism for sliding or rotating the four developing units into an operative position is also employed. This technology is described in, for example, Japanese Patent Application Laid-Open No. 2-189562 (1990) and Japanese Patent Publication No. 2-13304 (1990). However, the apparatus having this construction is complex and not easy to handle because of the mechanism required for driving the developing units.
On the other hand, a method is proposed in Japanese Patent Application Laid-Open No. 2-213884 (1990), where a photosensitive body is formed as a belt, and three or four developing unit are arranged to make use of the flat surface portion of the photosensitive belt.
However, it has been difficult for the conventional electro-photographic apparatuses to satisfy quality of image, speed of printing, easiness of maintenance and size of apparatus requirements at one time. There occurs any one or more of a degradation in the quality of the image, a lowering in speed of the printing and an increase in the size of the apparatus. Therefore, although various methods have been proposed, it has not been possible to provide a high-quality-image and small-size color electrophotographic apparatus having a size capable of use as a desk-top apparatus.